In order to ensure the availability of electrical loads in electrical systems of motor vehicles, especially in the case of applications that are critical to safety in motor vehicles, the energy storage mechanisms (also referred to as “energy stores”) for supplying these electrical systems are monitored continuously with respect to their performance and/or available load or energy, as well as with respect to the state of ageing and the state of soundness. The basic assumption for making available electrical energy by energy stores is a qualitatively high-grade galvanic connection to the wiring harness and the body sheet of the vehicle.
In order to ensure safety during vehicle operation, a complete loss of the energy store, e.g., a vehicle battery, in the vehicle has to be identified and indicated, e.g., within the scope of a warning report. The loss of the energy store, e.g., a motor vehicle battery, may be caused by several reasons. For instance, a breakage of the conducting connection between the negative pole and the vehicle body may have occurred; a breakage of the pole shoe or the cell connector in the battery may have occurred; the battery terminal clamp may have become detached from the battery terminal; or a cable starting from the positive clamp may have become detached. These occurrences may prevent the use of the energy potential in the energy store.
In methods known from the prior art for monitoring an energy store, e.g., a motor vehicle battery, measurements of the battery voltage, the current and the temperature are performed, using inductive or ohmic battery sensors.
According to one method described in published German patent document DE 101 42 085, a diagnosis of the electrical system of a motor vehicle is made, the electrical system voltage being recorded and its ripple curve being evaluated. A method described in published international patent document WO 98 205 94 provides that an interruption is detected in the connection between the energy store and the vehicle's electrical system. According to published international patent document WO 98 205 94, the impedance of the electrical system is evaluated and viewed from the point of view of the battery.
Both of the above-described methods assume frequent measurements of variables such as the clamp voltage and the clamp current, and require a calculation of the quotient of the absolute and/or differential voltage and current values. Accordingly, the above-described methods also require a relatively high calculating capacity, conditioned by a high scanning rate. According to the method described in published international patent document WO 98 205 94, the impedance of the entire electrical system of a vehicle is determined from the point of view of the energy store, i.e., the motor vehicle battery. An increase of the impedance in the electrical system of the motor vehicle above a threshold value is regarded as an indication of an interruption of the electrical connection between the energy store and the motor vehicle.
A method and a device for providing safety measures against failure of the vehicle electrical system are described in published German patent document DE 101 50 381. According to this published German patent document, the vehicle electrical system is fed by at least one battery and one generator. In order to guarantee the stability of the electrical system, the energy sources are constantly monitored, and when there is a defect in, or a strongly diminished performance of, one of the energy sources, an active intervention in the driving operation of the vehicle is made in order to avoid the complete failure of the effected components during travel. For this, the maximum rotary speed of an internal combustion engine of a vehicle may be limited, or the vehicle may be braked to a complete standstill.
Published German patent document DE 199 64 057 describes a method for detecting a defective motor vehicle battery. When the engine of the vehicle is running, the battery is charged by a generator to an increased voltage setpoint value that is pre-specified by a control unit, and the battery is subsequently discharged by an electrical load. The difference in voltage that comes about during charging and discharging within a time interval, i.e., the time interval that expires up to the reaching of two voltage threshold values, is evaluated as an indicator for the operating state of the battery, i.e., as an indicator for its storage performance. The evaluation takes place by recording the voltage gradient of a predefined curve that was previously ascertained in an empirical way. The result obtained is optically or acoustically indicated by an appropriate indicator.
Published German patent document DE 102 19 824 describes a method and a device for detecting an operation of an electrical system without battery-supplied energy. The electrical system includes: at least one generator driven by a motor; a battery; an electrical load connectible to the battery; and means for voltage evaluation. The generator voltage and the battery voltage are measured, and the voltage values ascertained are put in relationship to each other, and from the results obtained from this, one may conclude that there has been a malfunction of the battery or an interruption of the connection between the battery and the electrical system, and thus an occurrence of a battery-less operation. Both the ripple of the generator voltage and the ripple of the electrical system are ascertained, and the two ripples ascertained are put in relationship to each other. When there is a deviation of the ascertained ripple from pre-defined boundary values, this also leads to the conclusion that a battery-less operation is occurring. A device for carrying out the method described in the published German patent document DE 102 19 824 includes means for voltage evaluation, which in turn may include at least one band-pass and/or means for instantaneous peak value measurement and/or means for maximum and minimum determination of voltages. The means for the voltage evaluation measures both the generator voltage and the battery voltage.